In a general aspect, the present invention relates to a tire for vehicles provided with a tread having a low rolling resistance.
More particularly, the present invention relates to a tire for vehicles comprising at least one belt layer coaxially extending around at least one carcass ply, a composite tread coaxially extending around the belt layer and comprising a radially outer layer adapted to get in touch with the ground and a radially inner layer interposed between the radially outer layer and the belt layer.
In the field of tire production, the main object of an increasingly greater part of research is that of obtaining an optimum compromise between, on the one hand, the tire rolling resistancexe2x80x94which should be reduced to the minimumxe2x80x94and, on the other hand, the characteristics of handling and comfort of the tire.
This object is particularly difficult to achieve when the tire is provided with a tread of the composite type, i.e. a tread comprising a radially outer layer and a radially inner layer having mechanical and hysteretic characteristics different from each other.
This particular tread structure is, in fact, particularly affected by the conflict existing between the opposite hysteretic characteristics necessary to achieve the aforementioned object. In order to reduce the rolling resistance of the tire, in fact, it is necessary to provide a radially inner layer of the tread having a low hysteresis value (which may be related to the value of tang xcex4 at 70xc2x0 C.) and, as such, suitable to dissipate a limited amount of energy during rolling: in so doing, however, handling and comfort of the tire worsen, since these characteristics may be achieved by using rubber compositions having a high hysteresis value and, as such, adapted to dissipate an amount of energy suitable to ensure a high adhesion between the tread and the ground.
In order to limit this drawback, as described for instance in U.S. Pat. No. 4,319,619, it has been proposed to use a tread of the composite type comprising a radially inner layer, obtained by vulcanizing a rubber composition including natural rubber and/or diene rubbers, having a value of tango at 40xc2x0 C. not higher than 0.2 and a value of the modulus of elasticity Exe2x80x2 not lower than 120 kg/cm .
According to what is reported in the aforementioned patent and thanks to the use of a rubber composition having a low value of tang xcex4 and a high value of the modulus of elasticity in the radially inner layer of the tread, it is possible to reduce the rolling resistance of the tire without causing a worsening of the tire properties at high speeds or during running on a dry road.
However, the applicant has found that such a tread may give rise to a worsening of the comfort characteristics of the tire which dampens to a smaller extent the unevenness of the ground.
In order to obtain improved handling characteristics of the tires, the prior art has also proposed, as for instance described in Canadian patent CA 1 228 282, to use a composite tread comprising a radially inner layer, having a value of the modulus of elasticity E"" at 25xc2x0 C. comprised between 100 and 250 kg/cm2, and a radially outer layer, having a value of the modulus of elasticity E"" at 25xc2x0 C. comprised between 70 and 150 kg/cm2 and a value of tang xcex4 not lower than 0.25, wherein the ratio between the modulus of elasticity of the radially inner layer and that of the radially outer layer (measured at 25xc2x0 C.) is not lower than 1.15.
However, the Applicant has found that the tires made according to said Canadian patent, while substantially meeting the purpose, possess high hysteresis values and therefore a high rolling resistance.
The technical problem underlying the present invention is therefore that of providing a tire for vehicles provided with a tread of the composite type which would allow to obtain, with respect to the tires of the cited prior art, a better compromise between, on the one hand, the rolling resistance of the tire and, on the other hand, the characteristics Of handling and comfort thereof.
According to the invention, the aforementioned technical problem is solved by a tire for a vehicle comprising at least one carcass ply, at least one belt layer coaxially extending around the at least one carcass ply, and a composite tread coaxially extending around the at least one belt layer. The composite tread comprises a radially-outer layer adapted to contact the ground and a radially-inner layer interposed between the radially-outer layer and the at least one belt layer. A first ratio of a modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially-inner layer to a modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially-outer layer is greater than or equal to 1.1:1 and less than or equal to 3:1, and a second ratio of a loss tangent at 70xc2x0 C. of the radially-inner layer to a loss tangent at 70xc2x0 C. of the radially-outer layer is less than 0.8:1.
Surprisingly, the Applicant has found that in order to achieve the best possible compromise between rolling resistance, handling and comfort of the tire, it is not sufficient to optimize in a broad sense the hysteretic or modulus characteristics of the radially inner layer or those of the radially outer layer of the tread, but that it is necessary to provide that the values of the modulus of elasticity and tang xcex4 of the radially inner layer of the tread with respect to the radially outer layerxe2x80x94 measured at a specific temperature substantially equal to the running temperature of the tire in normal use conditions (70xc2x0 C.)xe2x80x94fall within a specific range of values.
More particularly, the applicant has found that to achieve the best possible compromise between rolling resistance, handling and comfort of the tire, it is necessary that:
i) the ratio between the value of the modulus of elasticity measured at 70xc2x0 C. of the radially inner layer and the corresponding value of the radially outer layer be comprised between 1.1 and 3, and that
ii) the ratio between the value of tang xcex4 at 70xc2x0 C. of the radially inner layer and the corresponding value of the radially outer layer be lower than 0.8.
By respecting these ratios between the values of modulus of elasticity and tang xcex4 at 70xc2x0 C. of the two layers of the composite tread, it has been found that it is not only possible to reduce the rolling resistance of the tire, but alsoxe2x80x94and quite surprisinglyxe2x80x94to achieve better handling and comfort performances with respect to those achieved by known tires for the same type of use.
Preferably, the ratio between the modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially inner layer and the modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially outer layer is comprised between 1.2 and 2.5 and, still more preferably, between 1.5 and 2.
Preferably, furthermore, the ratio between the value of tang xcex4 at 70xc2x0 C. of the radially inner layer and the value of tang xcex4 at 70xc2x0 C. of the radially outer layer of the tread is lower than 0.7 and, still more preferably, comprised between 0.2 and 0.5.
In this way it has been advantageously possible to achieve an optimum compromise between the rolling resistance of the tire and its handling and comfort performances.
Preferably, and in order to achieve the aforementioned ratios, the modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially inner layer of the tread is comprised between 5 and 14 MPa, while the modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially outer layer is comprised between 4 and 8 MPa.
More preferably, the modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially inner layer is comprised between 6 and 12 MPa, still more preferably between 6.5 and 9 MPa, while the modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially outer layer is comprised between 5 and 7 MPa.
By fulfilling the aforementioned values of the modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially inner layer, it has been found that it is also possible to gradually improve the handling and comfort performances of the tire, while keeping low the rolling resistance of the same.
Preferably, the value of tang xcex4 at 70xc2x0 C. of the radially inner layer of the tread is lower than 0.120 and, still more preferably, it is comprised between 0.03 and 0.06.
In this way, it is possible to achieve the lowest possible rolling resistance of the tire, with all the ensuing advantages, without however adversely affecting neither the handling nor the comfort of the same tire.
Preferably, the thickness of the radially inner layer of the tread is uniform; preferably, such thickness is greater than 1 mm and, still more preferably, it is comprised between 1.5 and 2.5 mm.
On the contrary, the radially outer layer of the tread should have a thickness at least equal to, and preferably greater than, the thickness of the grooves formed therein (usually 7-8 mm in motor-cars), not to allow the radially inner layer to get in touch with the ground when the radially outer layer is worn out.
Preferably, the thickness of the radially outer layer of the tread is selected so that the ratio between the thickness of the radially inner layer (selected from the aforementioned range of values) and the overall thickness of the tread is comprised between 0.01 and 0.7, preferably between 0.015 and 0.5.
For values of such thickness ratio lower than 0.01, the effect ensuing from the high modulus of elasticity at 70xc2x0 C. of the radially inner layer with respect to the modulus of the radially outer layer is not perceived and, as a consequence, the tire has lower handling characteristics. On the contrary, for values of such ratio higher than 0.7, the effect of the radially outer layer is not perceived and therefore the tire shows again lower handling characteristics.
For the purposes of the invention, the radially inner and outer layers of the tread may be obtained by forming and vulcanizing suitable rubber compositions (the composition of which is not per se critical for the purposes of the invention), adapted to achieve at 70xc2x0 C. the aforementioned desired values of Exe2x80x2 and tang xcex4.
Preferably, the radially inner layer of the tread is obtained by forming and vulcanizing a rubber composition comprising:
a) a cross-linkable unsaturated chain polymer base,
b) from 10 to 60 phr of at least one carbon black-based reinforcing filler,
c) an effective amount of at least one sulfur-based vulcanization agent.
In the following description and in the appended claims, the term: xe2x80x9cphrxe2x80x9d is used to indicate the parts by weight of a component of the rubber composition with respect to 100 parts by weight of the polymer base.
Preferably, the polymer base includes natural rubber and at least one elastomeric diolefin polymer obtainable by polymerizing, in solution or in emulsion, one or more conjugated diene monomers, possibly in admixture with an aromatic vinyl hydrocarbon, the latter being present in the polymer in an amount generally not higher that 50% by weight based on the total weight of the polymer.
Preferably, the elastomeric diene polymer contains 30 to 70% by weight of diolefin units having a 1,2 structure based on the total weight of the polymer.
For the purposes of the invention, the conjugated diene monomer of preferred use is selected from the group comprising: 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, and mixtures thereof, while the aromatic vinyl hydrocarbon of preferred use is selected from the group comprising: styrene, xcex1-methyl-styrene, p-methyl-styrene, vinyl-toluene, vinyl-naphthalene, vinyl-pyridine, and mixtures thereof.
In the following description and in the appended claims, the term: diolefin unit having a 1,2-structure, is used to identify the fraction of units deriving from a 1,2-polymerization of the conjugated diene monomer. If the conjugated diene monomer is 1,3-butadiene, said diolefin units having a 1,2-structure have the following structure formula: 
Preferably, the elastomeric diolefin polymer is selected from the group comprising styrene/1,3-butadiene copolymers (SBR), poly-1,3-butadiene (BR), styrene/isoprene copolymers and the like or mixtures thereof.
Additionally, the elastomeric diolefin polymer may be of modified type, i.e. xe2x80x9cterminatedxe2x80x9d and/or xe2x80x9ccoupledxe2x80x9d.
The term: xe2x80x9cterminatedxe2x80x9d copolymer, is used to indicate within the present description a copolymer obtainable by polymerizing the conjugated diolefin with a vinyl aromatic hydrocarbon in the presence of a metallorganic initiator and by subsequently reacting the intermediate copolymer so obtained with a suitable chain-terminating compound selected from the group comprising: substituted imines, an halogenated tin compound, at least one benzophenone compound having the following structural formula: 
wherein R1 and R2 are hydrogen, halogen, an alkyl, alkenyl, alkoxy, amino, alkylamino or dialkylamino group, and m and n are an integer from 1 to 10, either alone or possibly mixed with each other.
The term: xe2x80x9ccoupledxe2x80x9d copolymer, is used to indicate within the present description a copolymer obtainable by polymerizing the conjugated diolefin with an aromatic vinyl hydrocarbon in the presence of a metallorganic initiator and by subsequently coupling the polymer chains with a suitable chain coupling agent, comprising for instance an halogenated tin compound, with the primary aim of increasing the molecular weight of the copolymer so obtained.
In order to impart to the tread obtainable with the rubber composition of the invention adequate hysteretic characteristics, the polymer base preferably comprises 30 to 80 phr and, still more preferably, 40 to 70 phr, of natural rubber and 20 to 70 phr and, still more preferably, 30 to 60 phr of at least one elastomeric diolefin polymer as defined above.
Preferably, the rubber composition used to manufacture the radially inner layer of the tread comprises only carbon black as reinforcing filler, in an amount comprised between 10 and 60 phr and, still more preferably, between 30 and 50 phr.
The carbon black-based reinforcing fillers which may be employed in the rubber composition are those conventionally used in the art for tire treads and comprise, for instance, those indicated according to ASTM standards with the designations N110, N121, N134,N220,N231, N234, N299, N330, N339, N347, N351, N358 and N375.
For the purposes of the invention, sulfur or sulfur-containing molecules (sulfur donors) is the vulcanization agent of more advantageous use, with accelerators and activators well known to those skilled in the art.
Preferably, the amount of vulcanization agent based on sulfur or sulfur donors is comprised between 1 and 4.5 phr, still more preferably between 2 and 4.1 phr.
Preferably, the rubber composition of the radially inner layer of the tread further includes an effective amount of a suitable component adapted to increase the value of the modulus of elasticity at 70xc2x0 C. of the vulcanized rubber composition, without substantially increasing at the same time the value of tang xcex4.
For the purposes of the invention, such component may be selected from the group comprising one or more thermosetting resins, reinforcing fibers, for instance short fibrillated fibers of a suitable material, such as, for instance, glass, carbon, cellulose, polyvinyl alcohol, polyamide or polyester, or mixtures thereof.
For the purposes of the invention, the aforementioned thermosetting resin is preferably selected from the group comprising: resorcinol/methylene-donor compounds resins, epoxy resins, alkyd resins, and mixtures thereof.
Methylene donor compounds of preferred use are those selected from the group comprising donors of the amine type, such as for instance hexamethoxy methylmelamine (HMMM), hexamethylene tetramine (HMT), and mixtures thereof.
Epoxy resins of preferred use are those selected from the group comprising the resins of the epoxy/polyol, epoxy/diamine or epoxy/dicarboxy type.
Advantageously, such thermosetting resin contributes to further increase the modulus of elasticity at 70xc2x0 C. of the radially inner layer of the tread, allowing to use quantities of sulfur which are not prone to induce undesired reversion phenomena of the rubber composition.
Essentially, these phenomena consist in a partial breaking and/or cyclization of the sulfur-based cross links between the polymer chains of the rubber composition which may be substantially attributed to the high temperatures which are reached during the vulcanization operations and/or the use of the tire. This generally turns out in a reduction of the value of the modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the layer and therefore in a worsening of the handling of the tire.
For the purposes of the invention, the thermosetting resin is present in the rubber composition in a quantity adapted to achieve the desired values of the modulus of elasticity Exe2x80x2 at 70xc2x0 C., depending on the number of cross-linking groups present in the starting resin and/or on the nature of the same (either a two-component resin or a precondensed resin).
Preferably, the amount of thermosetting resin is comprised between 0.5 and 15 phr, more preferably between 2 and 10 phr and, still more preferably, between 3 and 7 phr.
The aforementioned reinforcing fibers preferably comprise the so-called aramid pulp (short fibrillated fibers of poly-paraphenylene-terephtalamide), of the type commercially known as xe2x80x9cKevlar(copyright)-pulpxe2x80x9d or xe2x80x9cTwaron(copyright)-pulpxe2x80x9d (Kevlar and Twaron are registered trademarks of DuPont and, respectively, of Akzo), described in U.S. Pat. No. 4,871,004, the description of which is herein incorporated by reference.
For the purposes of the invention, the aforementioned reinforcing fibers may be employed either as such or in the form of a predispersion in a suitable polymer matrix having the function of a carrier, for instance constituted by natural rubber, butadiene-styrene copolymers, ethylene-vinyl acetate copolymers or the like.
For the purposes of the invention, said reinforcing fibers are present in the rubber composition in a quantity adapted to achieve the desired values of the modulus of elasticity Exe2x80x2 at 70xc2x0 C. according to the chemical nature of the fiber, to the length/diameter ratio of the same and to the type of polymer base employed.
Preferably, the amount of reinforcing fibers (excluding the possible polymer carrier) is comprised between 1 and 6 phr and, still more preferably, between 1 and 4 phr.
In order to improve the handling characteristics of the tire, it is also preferable and advantageous to employ, in addition to the ingredients described hereinabove, 0.5 to 3 phr, more preferably 1 to 2.5 phr and, still more preferably, 1 to 2 phr, of at least one anti-reversion agent.
Advantageously, the additional presence of such anti-reversion agent contributes to improve the handling characteristics of the tire and to keep the same with time, reducing to a minimum said reversion phenomena of the rubber composition both during its vulcanization and during the use of the tire.
Anti-reversion agents of preferred use are those selected from the group comprising aromatic imide compounds, such as for instance 1,3-bis(citraconimidomethyl)benzene, commercially available under the trade name of Perkalink(copyright) 900 (Flexys, The Netherlands).
Additional or alternative anti-reversion agents may be selected from sulfur-containing agents adapted to stabilize the vulcanization lattice, such as for instance those selected from the group comprising: sulfur-containing silane compounds, dithiodimorpholine, dithiocaprolactame disulfide, and mixtures thereof.
In a preferred embodiment, the sulfur-containing stabilizing agent is a sulfur-containing silane compound having the following structural formula:
(R)3xe2x80x94Sixe2x80x94CnH2nXxe2x80x83xe2x80x83(I)
wherein:
R is an alkyl or an alkoxy group comprising 1 to 4 carbon atoms or a chlorine atom, n is an integer comprised between 1 and 6, and X is a mercapto group or a SmY group, wherein Y is (R)3xe2x80x94Sixe2x80x94CnH2nxe2x80x94, wherein R and n are as defined above, or a group selected from the following functional groups 
wherein m is an integer comprised between 1 and 6, and preferably is equal to 4, and R is an alkyl or an alkoxy group comprising 1 to 5 carbon atoms or a chlorine atom.
Still more preferably, the sulfur-containing silane compound is the silane-based coupling agent Si69 [bis(3-triethoxysilyl-propyl)tetrasulfide] (DEGUSSA) or the silane-based coupling agent [bis(3-triethoxysilyl-propyl)disulfide] (DEGUSSA) as such or in a suitable mixture with an inert filler (for instance carbon black), in order to facilitate its incorporation into the rubber composition, or the commercial products known under the trade names of X50S and X-75S (DEGUSSA) (50% carbon black, 50% silane).
In addition to the aforementioned ingredients, the rubber composition of the radially inner layer of the tread may incorporate one or more non cross-linking ingredients, known per se, necessary to impart the necessary mechanical and workability characteristics to the rubber composition.
Such ingredients are selected in particular from the group comprising plasticizers, working adjuvants, antioxidants, anti-aging agents, etc.
Each of such ingredients, furthermore, is selected in amounts and ratios which may be easily determined by a man skilled in the art.
For the purposes of the invention, the rubber composition employed to produce the radially outer layer of the tread may be constituted by conventional ingredients known in the art and including a silica-based reinforcing filler, such as those described in European patent application EP 0 728 803, the description of which is herein incorporated by reference.
Preferably, and in order to reduce to a minimum the rolling resistance of the tire, the vulcanized rubber composition employed to manufacture the radially outer layer of the tread comprises:
a) at least a first cross-linkable unsaturated chain polymer;
b) a first carbon black-based reinforcing filler;
c) at least a second cross-linkable unsaturated chain polymer;
d) a second silica-based reinforcing filler;
e) a suitable silica-coupling agent;
f) a sulfur-based vulcanization agent.
Preferably, the first cross-linkable unsaturated chain polymer is a modified copolymer obtainable by polymerizing at least one conjugated olefin with at least one aromatic vinyl hydrocarbon in the presence of an initiator comprising at least one metallorganic group and by subsequently reacting the intermediate copolymer so obtained with a compound comprising functional groups reactive with respect to metallorganic groups present on the copolymer and deriving from the initiator, said modified copolymer having a glass transition temperature comprised between 0xc2x0 and xe2x88x9280xc2x0 C., and comprising a total amount of aromatic vinyl hydrocarbon comprised between 5% and 50% by weight based on the total weight of the same.
Preferably, the second cross-linkable unsaturated chain polymer is, instead, a copolymer obtainable by polymerizing at least one conjugated olefin with at least one aromatic vinyl hydrocarbon, said second polymer having a glass transition temperature comprised between 0xc2x0 and xe2x88x9280xc2x0 C., and comprising a total amount of aromatic vinyl hydrocarbon comprised between 5% and 50% by weight based on the total weight of the same.
Such copolymers and the first carbon black-based reinforcing filler may be, for instance, those described above and constituting part of the rubber composition of the radially inner layer of the tread.
Preferably, the second silica-based reinforcing filler has a BET surface area comprised between 100 and 300 m2/g, a surface area measured by CTAB absorption according to ISO 6810 comprised between 100 and 300 m2/g, a DBP absorption value measured according to ISO 4656-1 comprised between 150 and 250 ml/100 g.
Coupling agents of preferred use are silane-based and correspond to the stabilizing agents of the vulcanization lattice of formula (I) described hereinabove.
According to a further aspect of the invention, a composite tread for vehicle tires is provided, in particular a premolded tread for covering worn tires having a good handling on wet roads and a low rolling resistance, comprising a radially-inner layer and a radially-outer layer wherein a first ratio of a modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially-inner layer to a modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially-outer layer is greater than or equal to 1.1:1 and less than or equal to 3:1, and a second ratio of a loss tangent at 70xc2x0 C. of the radially-inner layer to a loss tangent at 70xc2x0 C. of the radially-outer layer is less than 0.8:1.
According to a further aspect of the invention, a process is provided for manufacturing a tire for vehicle wheels, comprising the steps of providing at least one belt layer, circumferentially providing a tread comprising a radially-inner layer and a radially-outer layer around the at least one belt layer, and associating the tread to the at least one belt layer by vulcanization, wherein a first ratio of a modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially-inner layer to a modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially-outer layer is greater than or equal to 1.1:1 and less than or equal to 3:1, and a second ratio of a loss tangent at 70xc2x0 C. of the radially-inner layer to a loss tangent at 70xc2x0 C. of the radially-outer layer is less than 0.8:1.
According to a further aspect of the invention, a process is provided for covering a worn tire for a vehicle wheel, the worn tire having at least one belt layer, comprising the steps of: circumferentially providing a tread comprising a radially-inner layer and a radially-outer layer around the at least one belt layer of the worn tire, and irreversibly associating the tread to the at least one belt layer, wherein a first ratio of a modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially-inner layer to a modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially-outer layer is greater than or equal to 1.1:1 and less than or equal to 3:1, and a second ratio of a loss tangent at 70xc2x0 C. of the radially-inner layer to a loss tangent at 70xc2x0 C. of the radially-outer layer is less than 0.8:1
Lastly, according to a further aspect of the invention, a method is provided for reducing the rolling resistance and thereby increasing the wear resistance of a tire, comprising: providing the tire with at least one carcass ply, extending at least one belt layer coaxially around the at least one carcass ply, and extending a composite tread coaxially around the at least one belt layer, the composite tread comprising a radially-outer layer adapted to contact the ground and a radially-inner layer interposed between the radially-outer layer and the at least one belt layer, wherein a first ratio of a modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially-inner layer to a modulus of elasticity Exe2x80x2 at 70xc2x0 C. of the radially-outer layer is greater than or equal to 1.1:1 and less than or equal to 3:1, and a second ratio of a loss tangent at 70xc2x0 C. of the radially-inner layer to a loss tangent at 70xc2x0 C. of the radially-outer layer is less than 0.8:1.